Acceleration of T process assembly

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This MR does the same for the T process as !4713 (merged) and !4730 (merged) did for the ComponentTransport process:
adding the options <linear>
and <linear_solver_compute_only_upon_timestep_change>
to speed up the assembly.
The implementation is mostly copy & paste from ComponentTransport, the test cases are variations of an existing test case.
This MR also contains a bugfix

TimeLoop
now calls the processespreOutput()
hook always for the last timestep, even if no output is requested explicitely in the output config since the last timestep will always be output.  I also had to update the references for a 1D heat conduction test: There, the HeatFlowRates were strange (nonzero in the interior of the domain).
Verify effectiveness of the optimizations
$ cd <BUILDDIR>
$ ctest R ogs1D_HeatConduction_dirichlet
$ cd logs
$ grep cF "Assembly took" ogs1D_HeatConduction_dirichlet*.txt
ogs1D_HeatConduction_dirichlet_linear_dt_change.txt:500
ogs1D_HeatConduction_dirichlet_linear.txt:500
ogs1D_HeatConduction_dirichlet.txt:1000 < not optimized, two nonlinear iterations per timestep
ogs1D_HeatConduction_dirichlet_varying_dt_linear_compute_only_on_dt_change.txt:600 < second test case with varying time step size
ogs1D_HeatConduction_dirichlet_varying_dt_linear.txt:600
ogs1D_HeatConduction_dirichlet_varying_dt.txt:1200
$ grep cF "Eigen solver compute()" ogs1D_HeatConduction_dirichlet*.txt
ogs1D_HeatConduction_dirichlet_linear_dt_change.txt:1 < only one linear solver compute() call
ogs1D_HeatConduction_dirichlet_linear.txt:500 < one iteration per timestep and one compute() per iteration
ogs1D_HeatConduction_dirichlet.txt:1000 < two iterations per timestep and one compute() per iteration
ogs1D_HeatConduction_dirichlet_varying_dt_linear_compute_only_on_dt_change.txt:2 < one change in time step size = two compute() calls
ogs1D_HeatConduction_dirichlet_varying_dt_linear.txt:600
ogs1D_HeatConduction_dirichlet_varying_dt.txt:1200
Edited by Christoph Lehmann